Cerium oxide (ceria) has been reported for certain catalysis applications. For example, it has been described as the key oxygen storage component in three way catalysts (TWC) used for the treatment of automotive exhausts. It has also been described as being useful for the removal of sulfur accumulated on catalysts in fluid catalytic cracking in oil refineries or as a regenerable sorbent for H2S in hot-coal gas cleanup system. See Flytzani-Stephanopoulos, M.; MRS Bulletin, pp. 885-889 (2001); Fu, Q., Weber, A. and Flytzani-Stephanopoulos, M., Catalysis Letters, 77(1-3): pp. 87-95 (2001); Li, Y., Fu, O., Flytzani-Stephanopoulos, M., Applied Catalysis B: Environmental, 27: pp. 179-191 (2000); Bera, P., Aruna, S. T., Patil, K. C. and Hegde, M. S., Journal of Catalysis, 186: pp. 36-44 (1999); Bunluesin, T., Cordatos, H. and Gorte, R. J., Journal of Catalysis, 157: pp. 222 (1995); Diwell, A. F., Rajaram, K. R., Shaw, H. A., and Truex, T. J., Stud. Surface Science Catalysis, 71:139 (1991); Bunluesin, T., Gorte, R. J., Graham, G. W., Applied Catalysis B: Environmental, 15: pp. 107 (1998), Kundakovic, L. and Flytzani-Stephanopoulos, M., Journal of Catalysis, 179: pp. 203-221 (1998), and Tschöpe, A., Ying, J. Y. and Chiang, Y. M., Materials Science and Engineering A, 204:267-271 (1995).
Certain catalysts containing ceria have been described in Sk{dot over (a)}rman, B., et al., Journal of Catalysis, 211: pp. 119-133 (2002), and U.S. Pat. Nos. 5,993,762; 5,905,056; 5,733,837; 5,075,276; 4,996,180; 4,968,656; 4,839,146; 4,639,432; 3,819,535 and 3,284,370, as well as British Patent Nos. 609,166 and 127,609 for example. The above reported catalysts containing ceria have been prepared using techniques such as co-precipitation (CP), deposition/precipitation (DP), co-precipitation-gelation technique using urea (UGC), impregnation, magnetron sputtering process and/or combustion synthesis. Such processes and precursors have various drawbacks and shortcomings. For instance, several of these techniques involve lengthy processes of between 20-30 hours for catalyst preparation, which can be expensive and inefficient. Further, some of these processes are not easily scaled up.
For example, in the CP method, ammonium carbonate or sodium carbonate is used as the precipitant. In a typical procedure, an aqueous solution of cerium (III) nitrate and other metal nitrates are combined in desired proportions with (NH4)2CO3 at 60-70° C., at a constant pH value of 8. The precipitate is held at 60-70° C. for an hour, then filtered and washed with distilled water several times, followed by drying at 100-120° C. for 12 hours and calcining in air at several hundred degrees Celsius for 10 hours at a slow heating rate of 2° C./min. Sample preparation takes a total of about 22 hours. See Flytzani-Stephanopoulos, M.; MRS Bulletin, 885-889 (2001) and Liu, W., Flytzani-Stephanopoulos, M., The Chemical Engineering Journal, 64:283-294 (1996).
In the DP method, unlike the CP method, a catalyst support of doped or undoped ceria is independently prepared and calcined prior to use, and a desired amount of HAuCl4 is added dropwise into an aqueous solution of the precalcined ceria at a pH of 8. The subsequent workup-is similar to that of the CP method described above. The total preparation time is over 11 hours. See Fu, Q., Weber, A. and Flytzani-Stephanopoulos, M., Catalysis Letters, 77(1-3):87-95 (2001).
In a typical UGC method, Cu—CeO2 nanoparticles are co-precipitated from nitrate salts of metals with urea at about 100° C. The precursor salts used are metal nitrates (copper nitrates and ceric ammonium nitrates) and the cerium salt used is (NH4)2Ce(NO3)6. The preparation procedure basically consists of mixing aqueous metal nitrates with urea (H2N—CO—NH2), heating the solution to 100° C. under vigorous stirring with addition of deionized water, and boiling the resulting gel for 8 hours at 100° C. The product is then filtered and washed, followed by drying the residue in vacuum oven for 10-12 hours and calcining by slow heating in air for 4-6 hours. The total sample preparation time is about 28 hours. See Kundakovic, L. and Flytzani-Stephanopoulos, M., Journal of Catalysis, 179:203-221 (1998) and Li, Y., Fu, O., Flytzani-Stephanopoulos, M., Applied Catalysis B: Environmental, 27:179-191 (2000).
In a typical impregnation method, a CeO2 or oxide support is first independently prepared by precipitation or any other method such as heating cerium acetate in air. Then the CeO2 or metal oxide is mixed with a salt solution of copper and degassed under vacuum during impregnation. After excess solution is drained, the sample is dried for a few days at room temperature and then heated in air for four hours at 650° C. Liu, W., Flytzani-Stephanopoulos, M., Journal of Catalysis, 153:304-316 (1995).
In a typical magnetron sputtering process, nanocrystalline materials are generated in an ultra high vacuum chamber by magnetron sputtering from a mixed metal target of Cu—Ce in argon (30 Pa). The metal vapor is thermalized by the inert gas atmosphere and nucleated to form nanometer-sized clusters. The nanometer-sized clusters are collected on a liquid nitrogen cooled modified ground shield substance. After sputtering for 20 minutes, the ultra high vacuum chamber is evacuated and slowly back filled with oxygen to a final pressure of 1 kPa. The clusters are scraped and collected as loose powder, then calcined for 12 hours. See Tschöpe, A., Ying, J. Y. and Chiang, Y. M., Materials Science and Engineering A, 204:267-271 (1995). The magnetron sputtering process involves a physical synthesis process using an ultra high vacuum sputtering system, which is very expensive in terms of capital costs and maintenance. The product yield may also be low, and difficult to scale up.
In a typical combustion synthesis, ceric ammonium nitrate and copper nitrate are used as precursors of cerium and copper. Oxalydihydrazide (ODH, C2H6N4O2) prepared from diethyl oxalate and hydrazine hydrate is used as fuel. The combustion synthesis process involves mixing copper and cerium precursor salt with water and ODH fuel, introducing the mixture in a preheated muffle furnace at 350° C., boiling, foaming and igniting the solution, which is said to produce the nanopowder in five minutes. The nano-powder thus produced is heated at 300° C. for 12 hours to drive moisture out and stored in vacuum desiccators. See Bera, P., Aruna, S. T., Patil, K. C. and Hegde, M. S., Journal of Catalysis, 186:36-44 (1999). Such combustion synthesis methods appear to be hazardous.
Catalysts for low-temperature oxidation are described in, for example, U.S. Pat. Nos. 5,258,340; 5,017,357; 4,994,247; 4,956,330; 4,943,550; 4,940,686; 4,450,245; 4,426,319; 4,317,460; 4,256,609 and 4,252,687. However, these patents do not appear to describe nanocomposite copper-ceria catalyst.
Despite the developments to date, there is interest in low-temperature catalysts and methods for making such catalysts. Preferably, such methods could be used to produce large quantities of catalyst in relatively short periods of time and under non-hazardous conditions.